Although there is a growing awareness of the fundamental role that building ventilation plays on the health and well-being of occupants, many buildings currently suffer from poor ventilation i.e. insufficient fresh air renewal to dilute indoor pollutants. These pollutants include airborne virus particles such as SARS-CoV-2, influenza, or chickenpox, against which adequate ventilation is one of the primary mitigating measure[i]. In term of health impact, insufficient indoor air quality was estimated, for example, to cost €20 billion in France for the year 2004[ii].

The first factor put forward to explain the occurrence of ventilation issues is the voluntary or involuntary reduction of air renewal to decrease energy consumption. Indeed, air heating and cooling accounts for nearly 50% of the consumption of residential and non-residential buildings, which themselves account for 40% of total consumption in Europe and the United States[iii]. Other factors have been mentioned in recent large field studies in California schools, such as faults in installation, system maintenance or parameters setting for automatic ventilation control[iv].

Figure 1: Weekly evolution of the CO2 concentration measured with an eLichens sensor (30-min average) in two surface open spaces and a bedroom with one person absent during the day.

The best indicators to check the ventilation quality of a building is the measurement of CO2 concentration[1]. Concretely, high CO2 concentrations in a room indicates an insufficient rate of air renewal leading to accumulation of pollutants or undesirable substances and/or a number of person present higher than the initial design of the room. The level of 1000 ppm (1800 mg/m3) is generally reported to define optimal ventilation quality since it is cited in ASHRAE Standard 62 (American Society of Heating, Refrigeration, and Air

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[1] CO2 is an inert gas, present in outdoor air at levels of about 400 ppm and emitted either by combustion or by the respiration of organisms. The CO2 level in indoor air, except for specific controlled atmosphere, are higher or equal to the level in outdoor air.

Conditions Engineers)[i]. The Figure 1 illustrates high CO2 levels that may be observed in typical living environment like an openspace or a bedroom. The continuous monitoring of indoor CO2 level has been made possible in recent years with the significant technological progress achieved resulting in the production of miniature sensors at moderate cost. For this type of application, sensors based on infrared technology are widely preferred over other existing techniques due to their high sensitivity and stability both over time and in relation to changes in the environment. These sensors are also directly used for the control of HVAC systems allowing an optimized ventilation while minimizing energy consumption

To further improve the infrared sensors performances, solving various issues is required in order to obtain an accurate, small, and maintenance-free sensor, required for the mentioned applications. These limitations are:

   ¤ The large size of the sensor related to the length of the optical path directly proportional to the sensitivity.

   ¤ The problem of drift over time related, among others, to the aging of components,

   ¤ The complexity of packaging,

     ¤ The high cost of some components such as the light source.

In order to overcome these limitations, several technical solutions have been developed leading to several NDIR CO2 sensors with a variety of performances.

• The objective of this paper is to present the technological solution developed by eLichens to produce a miniature ultra-low power NDIR sensor, highly robust to environmental changes (and over time).  

The eLichens NDIR sensor family

With a key component that is a patented ultra-low power IR microsource developed in-house, eLichens has designed NDIR sensors, which are now recognized by leading companies in the security gas market.[i] Thanks to its know-how in design and mechatronics and in order to facilitate the integration of the sensors in the systems, several miniature shapes are proposed, including the standard "4-Series" format.

This sensors family differs from any other sensors by its high robustness over time and to changing environments with a measurement accuracy aligned with the recommendations of building & industrial standards[i]. This constitutes clear differentiators for the indoor air quality monitoring and control applications. To meet the application requirements, we have built a system around a durable source, a dual channel and optimized ageing of all components, as well as a calibration process, especially with respect to temperature, and a patented on-board recalibration algorithm.

An ultra-low power microsource with a long lifetime

The power and lifetime of the infrared source used in an NDIR sensor has a direct influence on the sensitivity and robustness of the gas measurement as well as the cost of the sensor. The table below compares the four types of infrared sources according to their optical power, price, and power consumption. Power consumption is a critical factor for the integration application in HVAC systems as well as a factor causing premature aging of the source and impacting the robustness of the sensor.

The IR source technology with the highest optical power is the laser but has the limitations of being an expensive component with high power consumption. In comparison, traditional incandescent lights are inexpensive but cumbersome with high power consumption and are practically no longer used in sensors with the advent of LEDs.

Advances in microelectronics have led to the design of a new type of source based on silicon wafers, which is particularly interesting for sensor fabrication due to the cost reduction associated with silicon technology and their very low power consumption.

-  Not suitable for this feature + relatively suitable  ++ well suitable  +++ very well suitable for this feature

The eLichens microsource, a suspended membrane, is the result of extensive research and development with the CEA-Leti. This microsource is protected by several patents.  Another main challenge, common to all MEMS components, is the industrialization of its fabrication. eLichens has set up a production proven manufacturing capability to build the microsource and to integrate it in its sensors.

Several key technological factors to ensure the stability of the measurements

The first key factor of the measurement reliability of NDIR sensors is the strategy to compensate for drift measurement that is certain. The two existing strategies are:

¤ Performing a reference measurement in parallel with the measurement of the gas concentration. The senor is then composed of a dual wavelengths system, one of which is outside the absorption range of the gas. To obtain this system, it is necessary that the sensor is equipped with two detectors and two light sources if the source emits only at a given wavelength (case of lasers and LEDs) constituting an additional cost.

¤ The implementation of an automatic recalibration algorithm whose principle is based on the assumption that the lowest CO2 concentration measurement encountered over a certain period of time corresponds to the level of CO2 concentration in outdoor air considered to be constant. The common drawback of this strategy is that there may be cases such as a building that is always occupied during a period when the lowest CO2 concentration measurement does not correspond to the ambient level and can lead to measurement inaccuracies.

eLichens opted for the strategy of combining these two solutions. The microsource, being a black body, only one source is required for the achievement of the dual-channel system. The contribution of the automatic recalibration algorithm is to provide the ability to compensate for any effects, such as changes in environmental conditions during transport, that might occur and affect the components thus the measurement.

Another factor strongly contributing to the accuracy of measurement but rarely mentioned has also been an extensive development at eLichens: the calibration process. Indeed, there is no standard machine or test bench to calibrate the sensors and this needs to be done in a reliable, and repeatable manner. For this purpose, the sensors need to be brought into contact with the gas to be calibrated in different concentrations within the defined measurement operating range (0 - 10,000 ppm for CO2). The high accuracy of the gas concentration (which is still a subject of study[i]) surrounding the sensor is crucial. This implies an airtight gas distribution system, a low gas volume to quickly reach the stabilization time between gas injections of different concentrations and a minimum number of manipulations to make the process highly repeatable.

eLichens has designed a patented system to meet these challenges. The sensor calibration and validation protocol also verify the power consumption and aging behavior of the sensors.

The combination of these key technological factors enables the successful creation of a reliable, miniature, and ultra-low power CO2 NDIR sensor. The figure below shows the repeatability and accuracy of the measurements obtained from 5 sensors compared to a so-called reference measurement made with a Vaisala sensor.

Figure 2: Comparison of CO2 measurements made by 5 eLichens sensors and a Vaisala sensor in an openspace during a weekend.

Conclusion

The different technological solutions presented here enable eLichens NDIR CO2 sensors to be robust over time and in changing environmental conditions. This feature is particularly relevant for air quality monitoring and control applications in closed environments.

While CO2 is the main indicator to monitor and control the ventilation of a building, there is a need for additional sensors to be aggregated in order to provide a complete experience with regard to health and well-being. Other toxic pollutants, such as particulate matters when

high concentrations (due to some indoor emission sources or an intake from outdoor air[i]) are another key indicator for air quality. eLichens has designed the eLsi station that integrates, in addition to the NDIR CO2 sensor, particle sensors, TVOCs and environmental parameters (temperature, humidity, sound and light intensity). The goal is to deliver a complete user-friendly solution for indoor air quality monitoring.

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[i] Iddon et al. CIBSE 2020.  The ventilation of buildings and other mitigating measures for COVID-19: a focus on winter 2020. Available on http://arvix.org/pdf/2009.12781.pdf

Li et al. 2007. Role of ventilation in airborne transmission of infectious agents in the built environment – a multidisciplinary systematic review. Available on https://europepmc.org/article/med/17257148

[ii] Boulanger et al. 2017. Socio-economic cost of indoor air pollution: A tentative estimation for some pollutants of health interest in France. Environ. Int. 104, 14-24. 

[iii] Cao et al. 2016. Building energy-consumption status worldwide and the state-of-the-art technologies for zero-energy buildings during the past decade. Energy Buildings, 128, 198-213

[iv] Chan et al. 2020. Ventilation rates in California classrooms: Why many recent HVAC retrofits are not delivering sufficient ventilation. Building and Environment, 167, 1062426.

   Fisk 2017. The ventilation problem in schools: literature review. Indoor Air, 27 (6), 1039-1051.

[v] Persely 2015 and literature review.  Challenges in Developing Ventilation and Indoor Air Quality Standards: The Story of ASHRAE Standard 62. Building and Environment, 91, 61-69.

[vi] www.businesswire.com/news/home/20200630005354/en/eLichens-to-powerHONEYWELL-BW-%E2%84%A2-SOLO-CO2-with-Cranberry-NDIR-CO2-1-Series-Gas-Sensor

[vii] The building codes ASHRAE Standard 62-2019 and EN 16798-1:2019 for Europe impose characteristics on the accuracy of CO2 sensors applied to HVAC systems.

[viii] For example: Budiman et al. 2017 Gravimetric dilution of calibration gas mixtures (CO2, CO, and CH4 in E Balance: toward their uncertainty estimation. AIP Conference Proceedings.

[ix] Ramalho et al. 2015. Association of carbon dioxide with indoor air pollutants and exceedance of health guideline values. Building Environment, 1, 115-124